Air‐Coupled Ultrasonic Measurements in Composites

Air‐coupled ultrasound is a non‐contact technique and has clear advantages over water‐coupled testing. Research of air‐coupled ultrasonics, especially using capacitance and micromachined transducers, has been extensively reported in the literature. This paper reports our experience of applying piezoceramic air‐coupled transducers for nondestructive evaluation of composites. The beam profiles of air‐coupled piezoceramic transducers, with and without apodization, were mapped out. The transmission of air‐coupled ultrasonic energy through composite plates of different thickness was measured experimentally; model calculation of the transmission coefficient, taking into account the frequency bandwidth of the transducer, agreed with the measurement results. The occurrence of diffraction phenomenon (“Poisson bright spot”) while imaging flaws in composite laminates was investigated. The resolution of scanned images obtained with air‐coupled transducers was investigated for different frequency, focusing, and apodization conditions.